BBio 242
Summer 2015
PhysioEx Exercise 5: Cardiovascular Dynamics – KEY
GRADING: 20 points total (5 points for each of the four activities).
Activity 1: Studying the Effect of Blood Vessel Radius on Blood Flow Rate
 Experiment: The basic setup is two glass beakers connected by a tube through which a blood-like fluid flows.
Pressure is applied to one of the beakers to get the fluid to flow through the tube into the other beaker. This is a
model of circulation in which the applied pressure represents the pumping action of the heart. In this particular
experiment, pressure, viscosity, and length are kept constant (at 100 mm Hg, 1.0, and 50 mm, respectively)
while tube radius is varied between 1.5 and 5 mm. The purpose of the experiment was to see how radius affects
flow rate.
 Prediction: Answers will vary.
 Results: The graph below shows the effect of radius on flow rate. Flow rate increases dramatically (nonlinearly)
as radius is increased. For example, increasing the radius by a factor of 2 (from 1.5 mm to 3 mm) causes a nearly
16-fold increase in flow (from 4.0 mL/min to 63.6 mL/min).
 Conclusion: The data are consistent with Poiseuille’s Law of Laminar Flow, which says that flow is proportional
to radius raised to the 4th power. Thus, small changes in vessel radius – as can be achieved by contraction or
relaxation of the smooth muscles surrounding a vessel – can cause large changes in flow.
Activity 2: Studying the Effect of Blood Viscosity on Blood Flow Rate
 Experiment: Same as above, except that pressure, radius, and length were held constant (at 100 mm Hg, 5 mm,
and 50 mm) while viscosity was varied between 1.0 and 8.0. Thus, the purpose of this experiment was to see
how fluid viscosity influences flow rate.
 Prediction: Answers will vary.
 Results: The graph below shows the effect of viscosity on flow rate. Flow rate is inversely proportional to fluid
viscosity: if the viscosity is doubled, the flow rate is cut in half.
 Conclusion: These data are also consistent with Poiseuille’s Law of Laminar Flow. The viscosity of blood can
change according to the hematocrit; a higher density of red blood cells makes the blood more viscous.
BBio 242
Summer 2015
Activity 3: Studying the Effect of Blood Vessel Length on Blood Flow Rate
 Experiment: Same as above, except that pressure, radius, and viscosity were kept constant (at 100 mm Hg, 3
mm, and 3.5, respectively), while length was varied between 10 and 40 mm. The purpose of the experiment
was to determine how length affects flow rate.
 Prediction: Answers will vary.
 Results: The graph below shows the effect of length on flow rate. The relationship resembles the relationship
between viscosity and flow rate in Activity 2 above; as length increases, flow rate decreases.
 Conclusion: Once again, the data are consistent with Poiseuille’s law. The impact of length on flow is not the
most interesting part of Poiseuille’s law, since the length of a given vessel does not vary from minute to minute
or month to month.
BBio 242
Summer 2015
Activity 4: Studying the Effect of Blood Pressure on Blood Flow Rate
 Experiment: Same as above, except this time the radius, viscosity, and length were fixed (at 5 mm, 3.5, and 50
mm, respectively), while pressure was varied between 25 and 200 mm Hg.
 Prediction: Answers will vary.
 Results: The graph below shows the effect of pressure on flow rate. The two are linearly proportional; as
pressure increases, the flow increases, with a constant slope. For example, doubling the pressure doubles the
flow rate.
 Conclusion: This experiment offers additional support for Poiseuille’s law. In a real cardiovascular system, the
blood pressure can be varied by adjusting the heart’s pumping action, the degree of constriction/dilation of the
blood vessels, and/or the volume of blood in the system.